Plasma gasification is a process which converts organic matter into synthetic gas, electricity, and slag using plasma. A plasma torch powered by an electric arc is used to ionize gas and catalyze organic matter into synthetic gas and solid waste (slag). It is used commercially as a form of waste treatment and has been tested for the gasification of biomass and solid hydrocarbons, such as coal, oil sands, and oil shale.
Process
A plasma torch uses an inert gas such as steam. The electrodes vary from copper or tungsten to hafnium or zirconium, along with various other alloys. A strong electric current under high voltage passes between the two electrodes as an electric arc. Pressurized inert gas is ionized passing through the plasma created by the arc. The torch's temperature ranges from 4,000 to 25,000 °F (2,200 to 13,900 °C). The temperature of the plasma reaction determines the structure of the plasma and forming gas. This can be optimized to minimize ballast contents, composed of the byproducts of oxidation: CO2, N, H2O, etc..
At these conditions molecular dissociation can occur by breaking down molecular bonds. The resulting elemental components are in a gaseous phase. Complex molecules are separated into individual atoms. Molecular dissociation using plasma is referred to as "plasma pyrolysis.
Feedstocks
The feedstock for plasma waste treatment is most often municipal solid waste, organic waste, or both. Feedstocks may also include biomedical waste and hazmat materials. Content and consistency of the waste directly impacts performance of a plasma facility. Pre-sorting and recycling useful material before gasification provides consistency. Too much inorganic material such as metal and construction waste increases slag production, which in turn decreases syngas production. However, a benefit is that the slag itself is chemically inert and safe to handle (certain materials may affect the content of the gas produced, however). Shredding waste before entering the main chamber helps to increase syngas production. This creates an efficient transfer of energy which ensures more materials are broken down.Yields
Pure highly calorific synthetic gas consists predominantly of Carbon monoxide (CO), H2, CH, among other components. The conversion rate of plasma gasification exceeds 99%. Non-flammable inorganic components in the waste stream are not broken down. This includes various metals. A phase change from solid to liquid adds to the volume of slag.Plasma processing of waste is ecologically clean. The lack of oxygen prevents the formation of many toxic materials. The high temperatures in a reactor also prevent the main components of the gas from forming toxic compounds such as furans, dioxins, nitrogen oxides, or sulfur dioxide. Water filtration removes ash and gaseous pollutants.
The production of ecologically clean synthetic gas is the standard goal. The gas product contains no phenols or complex hydrocarbons however circulating water from filtering systems is toxic. The water removes toxins (poisons) and the hazardous substances which must be cleaned.
Metals resulting from plasma pyrolysis can be recovered from the slag and eventually sold as a commodity. Inert slag is granulated. This slag grain is used in construction. A portion of the syngas produced feeds on-site turbines, which power the plasma torches and thus support the feed system. This is self-sustaining electric power.
Equipment
Gasification reactors operate at negative pressure and recovers both gaseous and solid resources.Advantages
The main advantages of plasma technologies for waste treatment are:- Clean destruction of hazardous waste,
- preventing hazardous waste from reaching landfills,
- no harmful emissions of toxic waste,
- production of clean alloyed slag which could be used as construction material,
- processing of organic waste into combustible syngas for electric power and thermal energy, and
- production of value-added products (metals) from slag.
Disadvantages
Main disadvantages of plasma technologies for waste treatment are:- Large initial investment costs relative to landfill and
- The plasma flame reduces the diameter of the sampler orifice over time, necessitating occasional maintenance.
Commercialization
Municipal-scale plasma gasification is used commercially for waste disposal in nine locations with five more projects in development. Sites for gasification facilities are often at landfills where recuperative landfill mining can return the landfills to their original states. Plasma arc gasification is a safe means to destroy both medical and other hazardous waste.In the Northeast of England in the United Kingdom plasma gasification technology is being implemented within the Northeast of England Process Industry Cluster(NEPIC) on Teesside by Air Products. This company is building two units to gasify societal waste to produce energy with the synthesis gas produced.
Military Use
The US Navy is employing Plasma Arc Waste Destruction System (PAWDS) on its latest generation Gerald R. Ford-class aircraft carrier. The compact system being used will treat all combustible solid waste generated on board the ship. After having completed factory acceptance testing in Montreal, the system is scheduled to be shipped to the Huntington Ingalls shipyard for installation on the carrier.SUBSCRIBERS - ( LINKS) :FOLLOW / REF / 2 /
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